You had lunch, you're back at your desk, and suddenly the afternoon feels like wading through mud.
Part of this is digestion — blood flow shifts toward your gut after a meal, which can leave you feeling duller for a while. Part of it is your body clock — most people have a natural dip in alertness in the early afternoon, lunch or no lunch, tied to the same rhythm that makes 3am hard for night shift workers.
A heavy, carb-dense lunch tends to make the dip worse. A short walk after eating, or a smaller lunch, can take the edge off — but for a lot of people, the dip happens regardless.
Some people time a bit of caffeine right after lunch rather than before, so it lands when the dip actually hits. Neuchu's 50mg-per-piece format makes that easy to do at a desk without another coffee run. You can find it here.